Fatal Error message on my pages

I have this message on several of my pages:

Fatal error: Class 'WP_Screen' not found in M:kmwebdevwp-contentthemesupfrontelementsupfront-this-postlibthis_post.php on line 539

I think I know why this happened.

I deleted the box containing the page content on these pages, but I don't know how to get them back...

I'm sorry. Please help!?